The popularity of capacitive-touch screens has been increasing since the introduction of smart phones and tablet PCs (personal computers). Capacitive-touch screens are becoming larger in size and there is an increasing demand on the responsiveness, resolution and intelligence of these screens.
A capacitive-touch screen is usually composed of an array of capacitance sensors (also called nodes) where each capacitance sensor 100 (see FIG. 1) contains an electrical parasitic capacitance CP (referred to as baseline capacitance thereafter). Making physical contact (e.g. a finger touch) with a capacitance sensor 100 will add a second capacitance CF (referred to as foreground capacitance thereafter) in parallel with CP such that the overall sensed capacitance CS developed for a touched sensor is CF+CP. Ideally, after measurement and calibration, the foreground capacitance CF can be extracted from the sensed capacitance CS (i.e. CF=CS−CP).
Contact with a capacitance sensor 100 can be detected when the calibrated foreground capacitance CF on specific node(s) is greater than a pre-determined threshold. Because the baseline capacitance CP of each individual capacitance sensor 100 on a capacitive-touch screen can be different from each other, the calculated foreground capacitance CF of each individual capacitance sensor 100 may be different, making it more difficult to sense when contact has been made. These differences in baseline capacitance CP may be caused, for example, by variances in the layout of the circuitry used to create a capacitance sensor or they may be caused by manufacturing variances in the process used to make a capacitive-touch screen.
In addition, the calculated foreground capacitance CF of a capacitance sensor is dependent on the operating state of the electronic device that contains a capacitive-touch screen. For example, the baseline capacitance CP can be different when a cellular phone is plugged in for charging versus when making a phone call. The capacitance CF may also change as the device ages or due to environmental effects. Other electronic device that use a capacitive-touch screen include a hand-held personal computer, a tablet personal computer, a portable personal computer, a monitor and a television.
An accurate calibration of baseline capacitance CP is useful for the accurate detection of contact made with a capacitive-touch screen.